Importance of aerosol vertical structure in satellite ocean-colour remote sensing

The vertical distribution of absorbing aerosols affects significantly the reflectance of the ocean-atmosphere system. The effect, due to the coupling between molecule scattering and aerosol absorption, is important in the visible, especially in the blue, and becomes negligible in the near-infrared. Differences between top-of-atmosphere reflectance obtained with distinct vertical distributions increase with the sun, and view zenith angle, and the aerosol optical thickness, and with decreasing scattering albedo, but are practically independent of wind speed. In atmospheric correction algorithms, these differences are directly translated into errors on the retrieved water reflectance. They may reach large values even for small aerosol optical thickness, preventing accurate retrieval of chlorophyll concentration. A method has been developed to estimate aerosol altitude from data in the oxygen A-band of the MERIS, and POLDER sensors. The method is sufficiently sensitive to improve retrievals of water reflectance and chlorophyll concentration in the presence of absorbing aerosols.